Treacherous
by innocentthing
Summary: It was so, so dangerous to fall in love with a vampire. But Fionna, the strong, brave - and evidently stupid human did anyway. Fiolee, rated T for some cursing and references to adult themes.
1. Quicksand

I lot of people liked my last fic, so when inspiration hit, I started writing right away. This one will have a few chapters, but will probably be complete in a few days :)

I'm not affiliated with Adventure Time or Taylor Swift.

* * *

_Put your lips close to mine_

_As long as they don't touch_

_Out of focus, eye to eye_

_'Till the gravity's too much_

_And I'll do anything you say,_

_If you say it with your hands_

_And I'd be smart to walk away-_

_But you're quicksand._

It was so, so dangerous to fall in love with a vampire. But Fionna always found herself running headfirst, fearlessly into danger, so it's not like this was anything new.

Of course, to Cake or Gumball, going out with that "bloodsucking creep" (they knew he never actually drank blood, why did they have to be so dramatic?) would be worse than fighting a dozen Liches at once. Which was totally blowing things out of proportion.

Not that Fionna and Marshall Lee were actually going out. After all, he was waaaaay out of her league for one thing, and for another, it didn't actually matter if they went out, though it _would_ be nice. But it wasn't like she needed to be with him to validate her existence or some junk like that. Even so, she did always wonder what it felt like to kiss him, to hold him – and not just in a headlock.

She'd never tell him about her crush anyway, not because she was afraid of rejection, but because she didn't want their friendship to get all weird if he felt one way or another. Anyway, he was always messing with her, trying to get her to say the "L" word to him, so if she ever actually _did_, he probably wouldn't believe her anyway (...would he?).

And, Glob, how he _loved_ to mess with her! Like this one time, for example-

"Nice, dude!" Fionna clapped after Marshall Lee had played one of his newest songs for her. He often came to play them for her to see what she thought – after all, she was the only musically inclined – meaning, she could sing well enough - person nearby (when Cake wasn't available, anyway. And Cake was a bit too critical anyway – apparently, dulcimers are _so_ much better than guitars? Pft. Whatevs.). She always complimented him – which only boosted his ego – but wasn't afraid to give him criticism when necessary.

"Ya think?" he asked, setting his axe guitar down, leaning it against the couch in the treehouse's living room that the two were sitting on. "The second verse wasn't too...I dunno, cliché?"

"Maybe a little..." Fionna said thoughtfully, "But like, it really had some good emotions goin' through it! It was really good, I dunno what else you want me to say." She shrugged.

Marshall smiled, but it quickly became a smirk. "What do I want you to say?" Uh-oh. She knew that tone. "Maybe I want you to say how strongly it made you feel," he came nearer to her, "Maybe I want you to tell me how it made you feel something for _me,_" that _stupid _smirk on his face grew as he came closer – now she was backed up, laying on the couch, and he was floating mere inches above her. "C'mon, Fi, didn't your heart beat just a little bit faster, because it made you think of me?" he laughed.

Ooh, he was one arrogant bastard. But...he was also a _correct_ arrogant bastard. Not that Fionna would admit it...she swallowed – his face was mere inches from hers, and oh, how she wanted to just punch that stupid, knowing smirk off his face! But she couldn't. Why? She'd done it before, why was this time...why was her heart pounding, oh Glob, her face was turning red, she could feel it. Their eyes were locked – was is possible for your heart to beat so fast that it stopped? Fionna was pretty sure that was what was going to happen to her right now, because judging from the proximity of their faces, she was pretty sure that he was going to kiss her.

Unless, of course, she kissed him first...

'No, no! He's just messing with you, like always!' she thought, 'He _wants_ you to react, he _wants_ you to do something stupid so he can laugh at you or whatever!' But at the same time, she didn't care. Even if it was some joke or whatever, her body seemed to be detached from her mind. It ached to be touched – to feel his hands on her waist and his lips on hers and -

This was too much. She was _actually _about to close the distance between their mouths (mostly for fear that her heart would stop if this continued on much longer) when suddenly, letting out a laugh, he just stopped floating, flopping down on top of her.

"OOF!" Fionna coughed as he got off of her, regaining his spot on the couch, "Jeeze, dude, ya trying to make me puke?"

"Sorry," he laughed, putting his hands being his head suavely, "The way you were looking at me, I couldn't think of any better way to take you by surprise than just dropping like that."

"Oh, no," Fionna smiled slyly, "I meant putting your face so close to mine."

Marshall crossed his arms and rolled his eyes. Fionna stuck out her tongue, and they both laughed.

'I really dodged a bullet there,' Fionna thought. 'Would I actually have kissed him? Stupid emotions!' she lightly punched herself in the head, trying to clear it.

"Uh...Fionna...you okay?" Oops. Marshall was still there, giving her a rather concerned look.

"Uh...Y-yeah...Just uh..." before Fionna could come up with an excuse, Marshall's hand shot out over her mouth.

"Shh!" he quietly exclaimed, "I hear Cake coming home! C'mon, quick, let's hide and scare her!" he pulled his hand away from her mouth and instead extended it out to her. She looked at him, then placed her hand in his, and he flew them to the closet.

Never would Fionna pull a lame prank like this on her sister otherwise. But she wouldn't leave Marshall hanging, either.

And together, in the closet, in the dark, she couldn't help her heart beating a little faster than usual.

It was so dangerous to fall in love with a vampire. But Fionna, the strong, brave, _stupid_ human did anyway.


	2. Friction

__Disclaimer: I'm not affiliated with Adventure Time or Taylor Swift.

* * *

_I can't decide if it's a choice,_

_Getting swept away_

_I hear the sound of my own voice_

_Asking you to stay_

_And all we are is skin and bones,_

_Trained to get along_

_Forever going with the flow -_

_But you're friction._

How Fionna had gotten involved with Flame Prince, she hardly knew. They _went_ out a few times, but they were never officially_ going_ out. And just as soon as it had begun, it ended. If you asked her why, she couldn't tell you. Neither could he. But both knew it was right to end it; they were never truly in love.

Why was it then, that Fionna sat at home that night, crying softly (as much as she hated to admit it)? Cake had been gone for the past several days, taking care of those kittens, so she didn't even know of Fionna's relationship woes. It was like a punch to the human's stomach every time she thought of Cake's family – where she had failed, her sister had more than succeeded. Cake had a family; Fionna had no one (romantically-speaking).

"Fionna! Oh my Glob, I just had a break through! Y'know that song that I've been working on? I finally figured it out! It – Fionna?" that stupid, damned vampire came through her window. It just so happened that the night he figured out the most perfect song he'd ever written was the same night Fionna's first relationship ever had ended.

"Go away," she was turned away from him, lying on her bed. Her voice sounded angry, but he could hear the sadness behind it. Nothing could fool vampire ears. He didn't know what was going on, but he did know that Fionna was _never_ sad. She hardly ever cried, or even frowned. Every time he saw her, she was laughing, smiling; when he burst into her home via the window that night, he'd expected to interrupt her sleep, not some break-down.

"Hey, you okay?" his voice softened, floating into the room. As much as Marshall hated to admit it, he had a soft spot for the girl. It bothered him on those few occasions when she _was_ sad.

"I'm fine," Fionna said, unconvincingly, sitting up and turning towards him, sniffing and wiping the tears off her face, blinking back any other tears that threatened to spill over. She was the fearless heroine of Aaa, damn it! Why should she be crying just because some stupid guy had decided they should stop seeing one another? "Hic!" her throat caught and she covered her mouth as the tears began again.

"Dude, you are _so_ not okay," Marshall informed her, sitting next to her on the bed. "I'm not blind; I can see that you're crying. So why don't you make things easier on yourself and just tell me? You know I'm just going to get it out of you anyway, so let's skip the arguing – you're hardly up for it anyway - and you just tell me how you're feeling."

Fionna looked at him trying _so _hard to hold back those damn tears, but they just kept falling down her cheeks, and before she knew it, she was hugging Marshall Lee, her face buried in the crook of his neck, her shoulders shaking from crying so hard. And she was pouring her heart out, telling him all about Flame Prince.

" - A-and, I mean, I never thought I'd care about what a guy thinks about me," she finished, "But I thought he liked me, y'know? It felt nice... Oh Glob, I'm so stupid!" she cried out, the tears starting anew.

"Shhhhhh," Marshall Lee said softly, rubbing circles gently on her back. "He obviously doesn't deserve you. Look, for a human, you're pretty amazing. You're not afraid of anything, and you protect Aaa – you're fearless and willing to take on anything. He'd be lucky to have you. It's his loss, really."

Fionna sniffed, hugging the vampire tighter, and mumbled something into his shirt.

"What was that?"

She pulled back a little. "Don't leave," she heard herself saying, "Please stay, at least for a little bit. When you're not busy being a jerk, you're actually pretty good at making me feel better..."

Marshall chuckled. "Sure," he said, pushing her hair away from her face so that the tears wouldn't make it matted and sticky, "Whatever you want, Fi."

Fionna felt her face flush a little through the tears that were slowing down. She wasn't sure if she was playing right into his hand or if she was actually in control of her feelings, but she didn't care. He was making her feel better, and that was all that mattered. She pressed her face against his chest once again, feeling herself beginning to drift off to sleep. Crying took more out of you than fighting a three-headed octopus monster – which she'd done, quite successfully! But now, her body was exhausted.

As her brain shifted into dream-mode, Fionna mentally thanked the universe for letting Marshall Lee find her in her time of need. They were best friends – of course he'd understand her. And he'd had his heart broken before – despite his denial, she'd seen his memories - so of course he'd know what to say. But this made everything more difficult for her, since _he_ was the one who she really loved. She knew it wouldn't work out – it was wrong-er than being with Flame Prince – but even though she knew just how dangerous and wrong it was, it felt so _right_, curled up in her bed with her head resting on his chest, even with the tears dried on her face.

Humans and vampires were not supposed to get along. One was supposed to be food for the other, like a mouse was to a cat. But being the only human left, Fionna defied nature; she unintentionally fell for the self-proclaimed vampire king – and she knew that loving him was wrong, but when was love ever right?


	3. Nothing Safe is Worth the Drive

**Before you read this chapter, I just want to give y'all some background info. The "night-giggles" is just a name that I made up, but I've had it happen to me where I essentially act drunk, even though I've had nothing to drink, because I'm so tired. I'm not sure if it's happened to anyone else, but yeah, I get pretty delusional (it may lead to "night blogging" on Tumblr). I just didn't want y'all to think it's something I made up for the sake of the story.**

I am not affiliated with Adventure Time or Taylor Swift.

* * *

_Two headlights shine through the sleepless night_

_And I will get you_

_And get you alone_

_Your name has echoed through my mind_

_And I just think you should,_

_Think you should know_

_That nothing safe is worth the drive_

_And I will follow you,_

_Follow you home_

Falling in love with a human was so very dangerous, especially when you were heir to the Night-O-Sphere. But in Marshall Lee's defense, he hadn't meant to! It started off as a running gag, a prank that was so easy to pull, he played it time and time again. But after spending time hanging out with the girl, he began to notice things he shouldn't. Her perfect complexion, her sweet smile, her nonchalant-ness. If you asked him what he liked about her, there were approximately two thousand, five hundred and three things that he could name, and that was just stuff off the top of his head.

Even her name was flawless. It wasn't too girly, but it wasn't too masculine either. It fit her perfectly. And if you asked him, he would completely deny occasionally sitting in his room, strumming his guitar and singing that name over and over. It echoed in his head, and he couldn't get it out.

But he knew that she could never love him. Besides the whole vampire thing, he was always such a jerk to her and her other friends, intimidating even – he was lucky to even call her his friend. If he truly loved this girl, he should stop being such a total dick – but it was just so cute, the way her entire face turned pink, the way her eyes momentarily suggested the rate of her heartbeat had increased.

But Marshall Lee, the cocky Vampire King, was always sure if he got Fionna the Human alone, he could prove his feelings, and she would just fall into his arms and become his. So he got her alone, time and time again – but every time, he chickened out - he, _heir to the Night-O-Sphere, _chickened out, and instead, played it off like one of his pranks. It pissed him off.

"Argh!" Marshall yelled, punching the wall of his bedroom. He was so angry at himself – since when had he been this much of a coward? This was just some stupid human he was thinking about, so who cared what she thought? She should be lucky to consider him her friend, never mind -

A knock on the door pulled Marshall from his thoughts. 'I swear to Glob,' he thought, floating toward the door, 'If that Gumwad is asking to borrow another cup of flour, I'm gonna -"

"Heeey, Marshall."

"Oh. Hey, Fionna," he greeted, and the girl in front of him yawned, and he checked the clock. "Uh, you do realize it's two in the morning?"

"Yeaaah," she swayed, "I'm like super tired, but I can't sleep for some reason? D'ya know what I mean?" She blinked twice as much as usual, suggesting that she was in fact, exhausted.

"Not really..." Marshall said, moving aside to let her in. "Wouldn't Cake have a better idea than me?"

Fionna laughed. "Ah ha ha! Who knows, dude, she's with her boyyyfriend," she giggled.

'Oh, Glob,' Marshall realized, 'She has a case of the night-giggles.' Getting the night-giggles was not unlike drinking too much alcohol, except rather than ingesting something harmful, the person simply hasn't slept enough (which was, in a way, harmful). They become slightly delusional, laughing way more than what was appropriate and drawing out words, sometimes even imagining things, like alien abductions and...

He had to get her to sleep before she became out of control.

"C'mon," Marshall said, picking Fionna up and bringing her upstairs, "You need rest. More than you know."

"Nah, I know," Fionna giggled again, "But I toldja, dude, I can't get to sleep!"

"Shut up," he muttered, placing her on top of his bed. He pulled the covers over her small figure, and almost as soon as her head hit the pillow, she was asleep. "Couldn't fall asleep, huh?" he said softly, nearly a whisper. Still, he looked at her affectionately. Even with her limbs taking over all of the bed that he was floating above, even with her mouth wide open, making slight snoring noises, she was pretty cute.

And so, he lowered himself, and softly kissed her forehead.

"Mmmm," the girl mumbled, turning in her sleep, and Marshall froze, afraid he'd woken her up. But he hadn't, and he relaxed. He floated, laying in the air, watching her sleep with a smile on his face. Before he knew it, his eyelids were drooping, and he was asleep.

* * *

Fionna slowly came out of sleep. Well, she was awake, but only barely – the kind of awake where her eyes weren't even completely open, her vision was fuzzy - once she woke up the next morning, she would remember it, but it would just feel like a realistic dream. She really _thought_ it was a dream when she found herself in Marshall Lee's bedroom – in his bed! - and saw him floating, asleep, next to her. Instinctively, she reached out, wrapping her arms around his oh-too-close body, and pulled him to her (this _was_ a dream, after all). He, still asleep, enveloped her in a hug as well, one arm reaching across her back so his hand was cradling her head. He unconsciously lowered himself onto the bed, and their bodies were pressed against one another, holding each other closer in their sleep. To her, she was having an amazing dream; he was so dead asleep that he had no idea what he was doing.

* * *

"EEP!" Fionna squeaked loudly the next morning, waking Marshall.

"Wu- What?!" his eyes popped open. And that's when he noticed. Him. Fionna. In bed. Holding each other so closely, there was absolutely no space between them. "Uh," he started lamely, "I can explain..."

"What...the...junk...?" she forced out. Then, suddenly, more wide awake, she pulled away sharply and whacked him in the head. "YOU'RE DISGUSTING!" Fionna cried, and before Marshall could wrap his head around what was happening, she had run down the stairs and out of the house towards her own home.

'Perfect...' he thought, mentally slapping himself. 'I couldn't have slept on the couch, could I? I just _had_ to watch her sleep...'

Fionna threw open the door to the treehouse, and Cake, who was at the stove cooking, turned around, surprised. "Fionna? What are you – aren't you upstairs sleeping? I just got back, thought I'd surprise you..."

"Cake!" Fionna exclaimed, catching her breath. Her sister motioned for her to sit down, which she did, and everything poured out.

"Mmhmm... Mmhmm... Gotcha, okay," Cake listened very carefully, when Fionna came to the end of her story. "Alright so let me get this straight. You got the night-giggles and, since I wasn't here, you for _some_ reason went to Marshall Lee's house, fell asleep in his bed (which I am not happy with, mind you), and woke up next to him?"

"Basically, yeah."

"But you still had all your clothes on, just like you did the night before?"

"Well, yeah..."

"Look, girl, as much as I don't like that guy, judging from what you told me, all the dude did to you was take you in and help you get to sleep. Most likely, he fell asleep 'cause he was tired from dealing with your night-giggles – and don't try to deny it, you know how you get when you haven't slept enough."

"Yeahhh..." Maybe her sister was right. Everything was hazy but she remembered going to his home, him carrying her upstairs... she couldn't remember anything inappropriate happening whatsoever, actually. Well, except for how they woke up, but that wasn't even that bad. In fact, when she thought about it, it was sorta nice...

"Fionna...why did you go to him?"

"'Cause you weren't here, Cake. Duh," Fionna said like it was the most obvious thing in the world.

"Well yeah," Cake replied in a similar tone, "But why not Gumball or someone less...sketchy?"

"Marshall's place closest to here?"

"Not really, it's pretty much the same distance. But judging from your tone, I'd say you knew that already."

"Oh...But why else would I have gone to him?" Fionna asked, and Cake could see that her sister was truly puzzled.

Cake sighed. "Look...Fi...You know I don't want to say this, but do you think you maybe, y'know, have a thing for Marshall?"

"What!?" Fionna exclaimed, but her bright red face gave her away, and Cake gave her a look that said 'Really? Don't even try to pretend because I can see right through you.' Now it was Fionna's turn to sigh. "I dunno Cake, I mean, I tried really hard not to, but I guess you're right. No, I _know _you're right. Oh, Cake, what do I do!?" Fionna hid her face in her arms.

"Relax, hun," Cake said, stretching her arms around the human's shoulders.

"How am I supposed to relax? I'm in l- I really, really like Marshall, even though he's a vampire and all bad and junk. I know I shouldn't, I'm sorry, I-"

"Fionna!" Cake exclaimed, "You should never be sorry for how you feel, you know that!"

"But you don't really like him..."

"Who cares what I like? I mean, yeah, he's not my favorite person in Aaa, but he's also not the worst. Look, Fionna, sometimes a little risk is worth it. I mean, look at me and Lord Mochro. His ancestors hated cats, but we're in love, and nothing can ruin that. Same goes for you and Marshall. If you love him – even if you just 'really like' him – you should give it a shot. If you play it safe, you'll never be happy."

Fionna smiled. "Thanks, Cake," she said, hugging the cat.

Suddenly, there was a knock on the door. Cake stretched her face out to the window to see who it was. "Ooooh, speak of the devil! You want me to give you two some privacy?" Fionna opened her mouth to answer, but before she could speak, Cake continued, "The answer is yes. Fi, when do I ever let you be alone with a guy? It'd be best to take advantage of my generosity," she winked, and stretched out the back window in a spiral formation, but not before calling back, "You'd better answer the door! He ain't gonna wait forever!"

Fionna tried to control her blush as she got up and walked over to answer the door.

It was dangerous to fall in love with a vampire. But Fionna was about to throw everything aside and tell Marshall Lee that she was nuts about him, even if it meant that nothing would be the same.

* * *

I hope everyone enjoyed this! The next chapter should be up tomorrow, & it will be the last!


	4. This Slope is Treacherous

This is the last chapter! Thanks to everyone who's been favorite-ing - feel free to review as well! ;)

I am not affiliated with Adventure Time or Taylor Swift.

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_This slope is treacherous;_

_This path is reckless_

_This slope is treacherous -_

_I, I, I like it_

_This hope is treacherous;_

_This daydream is dangerous_

_This hope is treacherous -_

_I, I, I like it..._

Marshall Lee stood outside of the treehouse. It had taken a bit of courage to follow Fionna back home after the events of the previous night had come crashing back to him. He understood why she had come to the conclusion that she did, but he also wanted to make sure she understood what had _really_ happened. If she still chose to hate him, that was her bizz, he probably deserved it anyway for picking on her for so long, but he just wanted to make sure all the cards were out on the table before she jumped to conclusions.

So he had followed her (the stupidly bright sun made sure he brought his – he loathed the word, it was so uncool - _parasol_), though he had stood outside for quite a while before gathering the courage to knock on the door. He knew he might be making a mistake, that perhaps it was better if she came to him, but he just couldn't sit and wait.

And so he was half-surprised when she actually answered the door.

"F-Fionna!" He said, taken aback.

"Uh, yeah dude, this is my house?" she avoided eye contact.

"Look, I just wanted to say, I'm -"

"I'm sorry, Marshall!" Fionna suddenly exclaimed. Now he was even more confused. "I know now that you didn't do anything to me last night, that you actually took care of me and stuff, so...I'm sorry for what I said this morning."

Marshall Lee stared at the small girl in front of him for a moment, trying to comprehend what was happening. _She_ was apologizing to _him. _Okay, this had to be a daydream or something. But it was too real to be his imagination... Either way, he had to say something.

"It's okay, Fi. I understand why you felt that way, though, to clear my name, I fell asleep floating above the bed; I have no idea why I woke up so close to you."

Fionna smiled ever so slightly, because she knew he wasn't making this up for once. But she wouldn't tell him of her "dream" - it was more... fun? interesting? that he didn't know for sure. She believed him. "You wanna come in?" she asked, stepping aside and holding the door open.

"Of course," he smiled, "Inside is a heck of a lot better than standing out here with this stupid parasol," he said, making a face and closing it as he came inside, out of the sun. "So does this mean you forgive me?"

"Of course I forgive you, ya goof," Fionna playfully pushed the vampire. "But uh, look," she suddenly turned serious, remembering Cake's words, "There's something I kind of want to talk to you about." She motioned him to sit on the couch; he complied, and she sat next to him. "Marshall Lee," she took a deep breath, and he knew something serious was about to go down; she rarely referred to him with his full name, and she was avoiding eye contact again. "Marshall Lee, you are one arrogant, terrifying vampire. You're a total jerk, and I'm pretty sure Cake will never not be completely un-afraid of you. You always mess with me, and laugh even when it's totes not funny. But despite all that, there's something about you that...I dunno. Maybe under all of that, you're not as bad as you pretend to be, maybe -"

Fionna was cut off in her heart-spilling speech as Marshall Lee's lips collided with hers. Her eyes shot open in surprise, and she almost pulled away...until she realized, this was what she _wanted, _after all. This was happening, now, it was real, and it wasn't a joke, and it was terrifying and amazing all at once. She hardly knew how to feel, so she let her instincts take control, and she felt her arms wrap around his neck, pulling him closer. But Marshall pulled away almost as soon as she had done so.

"Fionna, I think I love you," Marshall Lee said softly. "I know it's stupid, but I had to let you know."

"This isn't a joke?" Fionna asked warily.

Marshall smiled. "I get why you'd think that, but no, this isn't a joke this time. Or a prank. I really mean it, Fionna."

Fionna blushed. "Oh, ok, well that's good since y'know, I might maybe love you too."

"Fionna," Marshall said calmly, and Fionna looked at him, "If you blush any more, I won't be able to resist drinking that pretty pink from your face."

Of course, this only made her blush more. "S-So what, are we like, boyfriend and girlfriend now?" Hopefully saying something would reduce the burning in her cheeks.

Marshall Lee shrugged. "I guess so."

"Prince Gumball's gonna be mad. You know he doesn't like you."

"Pft. So?"

Fionna laughed. "Yeah, you're right, who cares what anyone thinks. I-" Fionna's stomach rumbled loudly, reminding her she hadn't had breakfast.

"Want me to make you breakfast, honey bunny?" Marshall Lee smirked.

Fionna deadpanned. "Dude. Don't make this weird."

"You know I'm going to, just to bother you. How long have you known me again?" Marshall smirked, and Fionna rolled her eyes. He laughed and kissed her on the forehead.

"I think there's some leftover spaghetti," she said, motioning to the fridge.

"Spaghetti? For breakfast?" he raised an eyebrow, feigning shock. Then he smiled. "Sounds perfect."

As she watched the vampire heat up the pasta, Fionna thought on all of the challenges waiting for the two. She knew they might not be together forever - she knew it was pointless to _hope_ for them to always stay together – but she couldn't help daydreaming about the wonderful possibilities. Maybe this could work out after all.

Dating a vampire was so, so dangerous – but that sense of danger just made Fionna like it even more.

* * *

Thanks again for reading! I hope everyone enjoyed this :) Keep an eye out for more from me - my next one might be Finn/Flame Princess, but I'm def doing more Fiolee too.


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